This is caused by chest width and or shape. Males and those with typical male development tend to have both wider and rounder shaped chests, the classic "barrel" shape makes breasts sit wide and face outwards, whereas typical female chest is narrower and has flatter shape so obviously it makes boobs point easily straight out.
The way to deal with this is a bra with good side support like BlackButterfly mentioned. Then there are also ways on promoting inner breast development, vacuum pump domes can be made to sit tight in the middle and thus aim inner breast more, maybe topical NBE stuff might help with that too? I think I've seen quite nice inner and upper development from it, but what made the big difference for me was the fat graft breast augmentation. Second round will be more of the same, but that is what closed the gap almost perfectly.
Note that my chest isn't barrel shaped, nor extremely wide either. When I'm really slim at smallest my band length measured at 33", typically I fluctuate between 36 to 38 so just slightly above typical female average. Despite this, due to my breast shape I had a gap in the middle for the longest time, it seems that just growing enough is the best way to deal with it, of course it can't change chest width and shape as much, but a really good bra and working on middle fullness with NBE methods is the way to go.
Look for bras with wide bands and extra side support panels on them. Also sewing extra piece of thick fabric on the sides of the cups can give extra push on the sides.